Method for forming self-aligned source and drain contacts using a selectively passivated metal gate

ABSTRACT

A transistor structure includes a semiconductor substrate with a first surface, a diffusion region at the first surface of the substrate, a sacrificial gate formed on the diffusion region, and insulating side walls formed adjacent to the sacrificial gate. A metal gate is formed by etching out the sacrificial gate and filling in the space between the insulating side walls with gate metals. Silicided source and drain contacts are formed over the diffusion region between the side walls of two adjacent aluminum gates. One or more oxide layers are formed over the substrate. Vias are formed in the oxide layers by plasma etching to expose the silicided source and drain contacts, which simultaneously oxidizes the aluminum gate metal. A first metal is selectively formed over the silicided contact by electroless deposition, but does not deposit on the oxidized aluminum gate.

BACKGROUND

Electroless plating is a technique conventionally used for self-alignedgrowth of gate metal contacts on a seed metal layer using lowresistivity copper (Cu). The process succeeds primarily because Cu doesnot oxidize in the electroless Cu plating bath. On the other hand,aluminum (Al) readily oxidizes in aqueous plating baths so anelectroless plating process has not yet been developed for a seed metallayer comprising Al. Al has a moderately higher resistivity than Cu, buta relative cost advantage. It is desirable to form self-aligned contactson the source and drain regions while avoiding the formation ofadditional metallization on metal gates.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are understood by referring to thefigures in the attached drawings, as provided below.

FIG. 1 shows a top view of a transistor layout according to oneembodiment.

FIG. 2 shows a cross-section A-A′ of the transistor layout of FIG. 1,following a replacement metal gate process step, according to oneembodiment.

FIG. 3 shows the cross-section A-A′ following an oxide etch for contactvia formation according to one embodiment.

FIG. 4 shows the cross-section A-A′ following formation of a metalcontact in the via by electroless plating according to one embodiment.

FIG. 5 shows the cross-section A-A′ following formation of either acarbon nano tube (CNT) or refractory metal contact layer over theelectroless plated metal contact according to one embodiment.

FIG. 6 shows the cross-section A-A′ following formation of a planarizedlayer of deposited oxide according to one embodiment.

FIG. 7 shows the cross-section A-A′ following formation of an interlayerdielectric layer etched and filled with metallization to contact adiffusion layer and contact the gates according to one embodiment.

Features, elements, and aspects of the invention that are referenced bythe same numerals in different figures represent the same, equivalent,or similar features, elements, or aspects, in accordance with one ormore embodiments.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A transistor structure having self-aligned selective metal contacts tosource and drain diffusions is provided. In one embodiment, thetransistor structure comprises a semiconductor substrate with a firstsurface, a diffusion region at the first surface of the substrate, asacrificial gate formed on the diffusion region, and insulating sidewalls formed adjacent to the sacrificial gate. A metal gate is formed byetching out the sacrificial gate and filling in the space between theinsulating side walls with metal. Silicided source and drain contactsare formed over the diffusion region between the side walls of twoadjacent aluminum gates. One or more oxide layers are formed over thesubstrate. Vias are formed in the oxide layers by plasma etching toexpose the silicided source and drain contacts, which simultaneouslyoxidizes the metal gate.

FIG. 1 shows a top view of a transistor 100 according to oneimplementation, where gate contacts are formed of a metal (i.e., metalgate 150), desirably by a replacement gate (RMG) process metal. In oneembodiment, a sacrificial poly gate is etched out followed by a blanketmetal gate fill on the wafer. The deposited metal is then polished toleave the metal in the gate region. After the polish, the top of themetal gate 150 is exposed, which may short to contacts to source anddrain (S/D) unless precautions are taken to form self-aligned S/Dcontacts without shorting to the exposed metal gate.

In the following, one or more embodiments are disclosed, by way ofexample, as applicable to gate contacts formed of aluminum (Al) orcopper (Cu). It is noteworthy, however, that in some embodiments anyother suitable metal or compound may be utilized instead. For example,gate contact material may comprise any other element or compound thatresists electroless metallization, while electroless deposition on othercontacts (e.g., S/D) succeeds. Accordingly, the scope of this disclosureshould not be construed as limited to Al or other exemplary material andembodiments disclosed herein.

FIG. 2 shows a cross-section A-A′ of a first intermediate stage 100 ofthe formation of the exemplary transistor of FIG. 1 following RMGformation of the metal gate 150. In certain embodiments, the metal gate150 may comprise Al or Cu. For a Cu gate, a process may be implementedto form self-aligned contact using electroless plating of sacrificialcobalt (Co) deposited on the metal gate 150. For an Al gate, theelectroless Co plating may not occur as the Al is readily oxidized inthe plating solution. This noted property for Al, in certainembodiments, may be advantageously used to selectively deposit materialson source/drain (S/D) contacts but prevent such deposition on an Algate.

Referring back to FIG. 2, device 100 may be formed on a substrate 110.Substrate 110 in one embodiment may comprise silicon, for example. Adiffusion region 120 may be formed at the surface of substrate 110 toalter conductivity properties. In one implementation, shallow trenchisolation (STI) regions 130 are formed surrounding the diffusion region120 to electrically isolate transistor 100 from adjacent transistors orelectronic device functionalities formed on substrate 110. Metal gates150 may be formed using an RMG process. In alternate embodiments, aprocess other than an RMG process, such as a subtractive process, may beused to form the metal gates 150.

In accordance with one aspect, metal gate 150 is bracketed by sidewallspacers 160. Sidewall spacer 160 may be formed of an insulator such assilicon nitride, for the beneficial purpose of resisting etchants thatact on oxide 180. Oxide 180 may comprise silicon dioxide in oneexemplary embodiment. It is noteworthy that in certain implementationsother material or combinations of different materials may be utilized toform sidewall spacer 160 and oxide 180 such that when an insulator suchas oxide 180 is etched, another insulator or side wall spacer 160 isnot. Diffusion region 120 may be contacted with raised S/D contacts 170that are treated to form a silicide to enable further deposition ofconductive material.

FIG. 3 shows the cross-section A-A′ of a second intermediate stage 200of the fabrication process following an oxide etch to form contact vias210 through oxide 180 to expose silicided raised source/drain (RSD)contacts 170. It should be noted that metal gates 150 may be exposed tosubsequent processes which are provided in further detail below. Etchingvias 210 in oxide 180 with a plasma oxide etch method will oxidize themetal gate 150, producing a thin insulating oxide layer.

FIG. 4 shows the cross-section A-A′ of a third intermediate fabricationfollowing formation of a metal contact 220 in the via 210 by electroless(EL) plating, for example. EL plated metal contacts 220 may be formed ofone or more of CoP, CoB, CoW, CoMo, CoWB, CoMoP, CoMoB, NiWP, NiWB,NiMoP, and NiMoB or other conductive metal alloys of Co and Ni, by wayof example. The inclusion of boron (B) and phosphorous (P) in theplating bath provides electrons to assist the EL plating process. Sincethe metal gate 150 is oxidized during oxide etch formation of via 210and is further oxidized in the EL plating solution, no Co may bedeposited on the metal gate 150.

In some exemplary embodiments, carbon nano-tube (CNT) bundles are grownvertically on substrates with Co or Ni as the catalyst. FIG. 5 shows thecross-section A-A′ of a fourth intermediate fabrication stage 400following formation of either a carbon nano tube (CNT) or metal contactlayer 410 over the electroless plated metal contact 220 according to oneembodiment.

In certain embodiments, a refractory metal (e.g., TiW) may be deposited,by a selective plasma deposition scheme, on metal contact 220. In oneimplementation, the refractory metal is deposited on metal contact 220,desirably, with a higher deposition rate than on oxide 180. The plasmaconcurrently deposits the refractory metal on the metal contact 220while acting to remove the refractory metal on the oxide 180. Thedeposition and plasma etch thus form an anisotropic selective depositionof the refractory metal on metal contact 220. Since the metal gate 150is passivated (e.g., in the form of aluminum oxide or aluminumfluoride), the refractory metal can be deposited on EL plated Co or Nion silicide RSD contacts 170. A proper selection of plasma species,comprising oxygen or fluorine, for example, may ensure that the metalgate 150 continues to be passivated during the refractory metaldeposition.

FIG. 6 shows the cross-section A-A′ of a fifth intermediate stage 500 offabrication of the transistor following formation of a planarized layerof deposited oxide, according to one embodiment. A layer of oxide may bedeposited over oxide 180 to surround (CNT) or metal contact layer 410,after which the surface may be planarized. Referring back to FIG. 1,gate contact 10 can be patterned and formed (e.g., adjacent to thediffusion region 120) at this point. A sequence of various processes,including photomasking, oxide etching, and metal deposition may beemployed to form gate contact 10. In one embodiment, CNT layer 410 maybe etched out and replaced by the same metal contacting the gates duringthe deposition process for forming the gate contacts 10.

FIG. 7 shows the cross-section A-A′ following formation of an oxide 184(e.g., interlayer dielectric layer (ILD)) over oxide 182, which may beetched and filled with final contact metallization 190 to contact thediffusion layer 120 by way of contact layer 410, metal contacts 220 andsilicided source and drain contacts 170. The same oxide 184 may be, forexample, patterned and etched to enable formation of contacts to thegates 150 by etching a region of oxide layers including one or more of180, 182 and 140 opposite diffusion layer 120, as shown in FIG. 1. Thecontact metal 190 in one embodiment may be comprised of copper ortungsten.

For simplification, it is appreciated that the methods and structuresdisclosed herein may not require replacement of a metal by an insulatorto protect the gate, as the gate oxide layer that forms on Al may besufficient to prevent electroless deposition thereon while disposingmetal on other regions containing catalyst atoms to support deposition,such as, for example, electroless plating.

The various embodiments described above have been presented by way ofexample and not by way of limitation. It should be understood that theprocesses, methods, and the order in which the respective elements ofeach method are performed are purely exemplary. Depending on theimplementation, they may be performed in a different order or inparallel, unless indicated otherwise in the present disclosure.

The method as described above may be used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case, the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multi-chip package(such as a ceramic carrier that has either or both surfaceinterconnections of buried interconnections).

In any case, the chip is then integrated with other chips, discretecircuit elements, and/or other signal processing devices as part ofeither (a) an intermediate product, such as a motherboard, or (b) an endproduct. The end product can be any product that includes integratedcircuit chips, ranging from toys and other low-end applications toadvanced computer products having a display, a keyboard or other inputdevice, and a central processor.

Having thus described in detail embodiments of the present invention, itis understood that the invention defined by the appended claims is notto be limited by particular details set forth in the above description,as many apparent variations thereof are possible without departing fromthe spirit or scope thereof.

1. A method of forming a transistor structure comprising: providing asemiconductor substrate having a first surface; providing a diffusionregion at the first surface of the semiconductor substrate; forming asacrificial gate over the diffusion region; forming insulating sidewalls adjacent to the sacrificial gate; forming silicided source anddrain contacts over the diffusion region adjacent to the side walls;forming a first oxide layer over the substrate; forming an aluminum gateby etch removal of the sacrificial gate and filling a space formedbetween the insulating side walls with aluminum; applying a plasma etchto form at least one via in the first oxide layer to expose thesilicided source and drain contacts and passivate said aluminum gate;and selectively depositing a first metal layer in the via and not onsaid passivated aluminum gate.
 2. The method of claim 1, wherein theside walls are insulators with etch properties that are selectivelydifferent from the first oxide layer and the sacrificial gate.
 3. Themethod of claim 2, wherein the plasma oxide etch produces an oxide orfluoride on the aluminum gate.
 4. The method of claim 3, wherein thefirst metal layer is selectively deposited on the silicided raisedsource and drain contacts using electroless plating.
 5. The method ofclaim 4, wherein the first metal layer comprises at least one of CoP,CoB, CoW, CoMo, CoWB, CoMoP, CoMoB, NiWP, NiWB, NiMoP, NiMoB, or acombination thereof.
 6. The method of claim 4, wherein the oxide orfluoride formed on the aluminum gate prevents electroless plating of thefirst metal layer on the aluminum gate.
 7. The method of claim 6,further comprising selectively depositing a second metal layer over thefirst metal layer.
 8. The method of claim 7, wherein the second metallayer comprises vertically grown carbon nano tube bundles.
 9. The methodof claim 7, wherein the second metal layer comprises a refractory metal.10. The method of claim 9, wherein the second metal layer is depositedby plasma deposition, wherein the plasma substantially inhibitsformation of the second metal layer on oxides or fluorides, includingthe oxided or fluorided aluminum gate.
 11. The method of claim 7,further comprising: depositing a second oxide layer over the secondmetal layer; and planarizing the second oxide layer to expose the secondmetal layer.
 12. The method of claim 11, further comprising: forming asecond via in the second oxide layer to gain access to the aluminumgate; and filling the second via with metal to form an interconnect tothe aluminum gate.
 13. The method of claim 12, further comprising:forming a third via in the second oxide layer to gain access to thesecond metal layer; and filling the third via with metal to form aninterconnect to the second metal layer.
 14. The method of claim 11,wherein the second oxide layer comprises vertically grown carbon nanotube bundles, the method further comprising: removing the verticallygrown carbon nano tube bundles from the second oxide layer, wherein theremoving forms a hole in the second oxide layer providing access to thefirst metal layer; and depositing a metal in the hole in the secondoxide layer to form a contact to the first metal layer.
 15. A method offorming a semiconductor structure, comprising: forming a metal gate on asemiconductor substrate; forming silicided source and drain contacts onthe semiconductor substrate; forming a first dielectric layer over thesilicided side source and drain contacts; etching the first dielectriclayer to form vias to expose the silicided source and drain contactsthen; passivating the metal gate to form a passivated metal gate; andselectively depositing a first metal layer into the vias but not on thepassivated metal gate.
 16. The method of claim 15, wherein the metalgate comprises Al.
 17. The method of claim 15, wherein passivation ofthe metal gate occurs by plasma etch.
 18. A method of forming asemiconductor device, comprising: forming a metal gate on asemiconductor substrate having a pair of nitride sidewall spacers;forming silicided source and drain contacts on the semiconductorsubstrate adjacent to said nitride sidewall spacers; forming a firstoxide layer over said nitride sidewall spacers; plasma etching the firstoxide layer to form vias to expose the silicided source and draincontacts and passivate the metal gate; selectively depositing a firstmetal layer into the vias but not on the metal gate.
 19. The method ofclaim 18, wherein depositing of the first metal layer is achieved byelectroless plating.
 20. The method of claim 18, wherein the metal gatecomprises Al, and wherein the plasma etch forms an oxide or fluoridepassivation layer on the metal gate.